The global increase in CO2 concentrations results in the reduced opening of the stomata. Although reduction in the stomatal opening in response to drought relies on abscisic acid (ABA), less is known about the mechanism controlling stomatal aperture in response to elevated CO2. Hsu et al. examined stomatal CO2 responses in Arabidopsis using mutant lines in ABA biosynthesis and perception. While ABA signal transduction accelerated stomatal responses, mutants in ABA perception were still able to respond to increased CO2 levels. The activation of SnRK2.6/OST1 kinase was observed during the ABA treatment, but not in response to elevated CO2. The guard cell S-type anion channels were activated by CO2 in the ABA-insensitive mutants, suggesting an ABA-independent pathway for CO2-induced stomatal closure. (Summary by Magdalena Julkowska) PNAS 10.1073/pnas.1809204115